From what I can read the flight does NOT have to bee booked on Dl.com. Yes I received this also
Terms & Conditions
Eligibility:
Only members who receive this communication directly from Delta Air Lines are eligible to participate in this offer. This offer is non-transferable. To participate in this offer, eligible members must register online by June 30, 2008.
Bonus Offer:
Purchase and fly round trip on a Delta marketed/operated flight between June 1, 2008 and July 31, 2008 and earn your choice of double bonus miles or double Medallion Qualification Miles (MQMs) on your first qualifying flight. Delta marketed codeshare flights are excluded from this offer. Bonus miles do not count toward Medallion or Million Miler™ status.
Travel Period:
Travel must be between June 1 and July 31, 2008. All travel must be completed by July 31, 2008.
Additional Restrictions:
Bonus miles or MQMs will post to the SkyMiles account of the qualifying member 4-6 weeks following the end of the promotion (July 31, 2008). Taxes and fees for Award Travel are the responsibility of the passenger and must be paid at the time the ticket is booked. Award Travel seats are limited and may not be available on all flights or in all markets. Hotel and rental car additions are subject to the terms and conditions of the service provider. All SkyMiles program rules apply. To review the rules, please visit delta.com/memberguide. Offer void where prohibited by law. Other restrictions may apply. Offer subject to change. Please refer Delta customer service representative to 970633.